1. Field of the Invention
The invention relates to a method and apparatus for quenching a gas stream in the production of vinyl chloride monomer and, more particularly, a method and quench column for quenching a gas stream comprising vinyl chloride, hydrogen chloride, and unreacted 1,2-dichloroethane.
2. Description of the Prior Art
Few monomers are commercially produced in the world on so large a scale as vinyl chloride monomer ("VCM"). Over 95% of the VCM produced in the world is made by pyrolysis of 1,2-dichloroethane, or ethylene dichloride ("EDC"), in accordance with the following reaction: ##STR1## This reaction typically takes place at elevated temperatures in the range of 900.degree. F.-1000.degree. F. within a cracking furnace having a plurality of pyrolysis furnace tubes. The EDC is thermally cracked to produce equal molar quantities of vinyl chloride and hydrogen chloride ("HCl"). Typically the cracking furnace is designed to convert 50% to 60% of the EDC feed per pass of the feed through the cracking furnace. Hot cracking furnace effluent gases containing vinyl chloride, hydrogen chloride, unreacted EDC, and undesired byproducts, including carbonaceous impurities, such as coke, and chlorinated organics, must be rapidly cooled in a quench column to stop the pyrolysis reactions and to minimize the formation of additional byproducts. Typically, in the prior art, these effluent gases are cooled by direct contact with cooled liquid EDC and vinyl chloride in the quench column.
A major portion of a VCM manufacturing plant's capital and operating costs are associated with the separation and purification of the cracking furnace effluent gas components. Fractional distillation processes are typically used to separate VCM and HCl products, and are used to remove undesired impurities from unreacted EDC for recycling to the cracking furnace feed stock.
There are many disadvantages associated with current quench column designs and methods of quenching utilized in the production of VCM. One disadvantage is the necessity to use relatively large capacity circulation pumps and piping, which result in substantial capital costs in constructing a VCM plant. There are process safety and environmental concerns resulting from circulated liquid containing undesired impurities, such as coke particles, which can cause severe pump maintenance problems and piping erosion. Historically, cracking furnace quench circulation piping failures have been a major source for VCM production plant fires. Another disadvantage associated with prior art quench column designs and methods of quenching is that it is difficult to remove coke particles, which causes relatively high fouling rates on VCM purification equipment and reduces equipment life. A further disadvantage associated with prior art quench column designs and methods of quenching is that poor component separation is achieved, which results in higher energy costs for the conventional downstream hydrogen chloride and vinyl chloride distillation process. An additional disadvantage of conventional methods and apparatus for quenching the cracking furnace effluent gas stream is that removal of solids from the quench column, such as coke particles, is typically done by filtering quench liquid which is removed from the bottom of the quench column. Filtering efficiency is typically very poor because conventional methods and apparatus tend to break up fragile coke particles into smaller particles which can pass through the filtering device.
Accordingly, prior to development of the present invention, there has been no method and apparatus for quenching a gas stream from an EDC cracking furnace which: does not require relatively large capacity circulation pumps and piping and their substantial capital costs; does not have process safety and environmental concerns arising from circulated liquid containing coke particles which can cause severe pump maintenance problems and piping erosion, and could lead to piping failures and resulting plant fires; easily permits the removal of coke particles from the quench column, whereby vinyl chloride purification equipment does not have relatively high fouling rates, which reduces equipment life; achieves good component separation, which results in lower energy usage for the downstream hydrogen chloride and vinyl chloride distillation process; and has substantial filtering efficiency to remove fragile coke particles. Therefore, the art has sought a method and apparatus for quenching a cracking furnace effluent gas stream which: does not require the use of relatively large capacity circulation pumps and piping, thereby having lower capital costs; does not have process safety and environmental concerns resulting from circulated liquid containing coke particles, thereby reducing pump maintenance problems and piping erosion; permits the easy removal of coke particles to avoid high fouling rates on vinyl chloride purification equipment, and increases equipment life; provides good component separation, which results in lower energy usage for the downstream hydrogen chloride and vinyl chloride distillation process; and achieves high filtering efficiency for the removal of coke particles.